Compensating for chromatic dispersion in optical fibers

ABSTRACT

An optical chromatic dispersion compensator ( 60 ) betters optical communication system performance. The dispersion compensator ( 60 ) includes a collimating means ( 61 ) that receives a spatially diverging beam of light from an end of an optical fiber ( 30 ). The collimating means ( 61 ) converts the spatially diverging beam into a mainly collimated beam that is emitted therefrom. An optical phaser ( 62 ) receives the mainly collimated beam from the collimating means ( 61 ) through an entrance window ( 63 ), and angularly disperses the beam in a banded pattern that is emitted from the optical phaser ( 61 ). A light-returning means ( 66 ) receives the angularly dispersed light and reflects it back through the optical phaser ( 62 ) to exit the optical phaser near the entrance window ( 63 ) thereof.

This application is a continuation of U.S. reissue patent applicationSer. No. 11/974,877, filed Oct. 15, 2007 now abandoned as a reissuepatent application of U.S. Pat. No. 7,099,531 that issued Aug. 29, 2006.

CLAIM OF PROVISIONAL APPLICATION RIGHTS

This application claims the benefit of U.S. Provisional PatentApplication No. 60/396,321 filed on Jul. 16, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the technical field of fiberoptic communication, and, more particularly, to compensating forchromatic dispersion that accumulates as light propagates through acommunication system's optical fiber.

2. Description of the Prior Art

Increasing demand for low-cost bandwidth in optical fiber communicationsystems provides motivation for increasing bothbit-rate/transport-distance, and the number of wavelength-divisionmultiplexed (“WDM”) channels which an optical fiber carries. A principallimiting factor in high bit-rate, long-distance optical communicationsystems is chromatic dispersion which occurs as light propagates throughan optical fiber. Chromatic dispersion causes a light wave at oneparticular wavelength to travel through an optical fiber at a velocitywhich differs from the propagation velocity of a light wave at adifferent wavelength. As a consequence of chromatic dispersion, opticalpulses, which contain multiple wavelength components, becomesignificantly distorted after traveling through a sufficiently longoptical fiber. Distortion of optical pulses degrades and losesinformation carried by the optical signal.

Chromatic dispersion of optical fibers can be characterized by two (2)parameters:

-   -   1. a group velocity dispersion (“GVD”) which is the rate of        group velocity change with respect to wavelength; and    -   2. a dispersion slope which is the rate of dispersion change        with respect to wavelength.        For a typical optical fiber communication system carrying a        broad range of wavelengths of light, such as a WDM system or        systems with directly modulated lasers or Fabry-Perot lasers, it        is necessary to compensate both for GVD and for dispersion slope        across the entire range of wavelengths propagating through the        optical fiber.

Over the years, several different types of optical fibers each of whichexhibits different chromatic dispersion characteristics have been usedin assembling optical communication systems. The dispersioncharacteristics exhibited by these different types of optical fibersdepend on the length of an optical fiber, the type of optical fiber, aswell as how the optical fiber was manufactured, cabling of the opticalfiber, and other environmental conditions. Therefore, to compensate forchromatic dispersion exhibited by these various different types ofoptical fibers it is desirable to have a single type of chromaticdispersion compensating device which provides variable GVD anddispersion slope to thereby simplify inventory control and opticalcommunication network management.

Several solutions have been proposed to mitigate chromatic dispersion inoptical fiber communication systems. One technique used in compensatingfor chromatic dispersion, shown schematically in FIG. 1A, inserts arelatively short length of a special dispersion compensation opticalfiber (“DCF”) 31 in series with a conventional transmission opticalfiber 30. The DCF 31 has special cross-section index profile andexhibits chromatic dispersion which opposes that of the optical fiber30. Connected in this way, light, which in propagating through theoptical fiber 30 undergoes chromatic dispersion, then propagates throughthe DCF 31 which cancels the chromatic dispersion due to propagationthrough the optical fiber 30. However to obtain chromatic dispersionwhich opposes that of the optical fiber 30, the DCF 31 has much smallermode field diameter than that of the optical fiber 30, and therefore theDCF 31 is more susceptible to nonlinear effects. In addition, it isdifficult to use a DCF 31 operating in its lowest spatial mode forcomplete cancellation both of GVD and of dispersion slope exhibited bytwo particular types of optical fibers, i.e. dispersion-shifted opticalfibers (“DSF”), and non-zero dispersion shifted optical fibers (“NZDF”).

An alternative inline chromatic dispersion compensation technique, shownschematically in FIG. 1B, inserts a first mode converter 33, whichreceives light that has propagated through a length of the first opticalfiber 30, between the first optical fiber 30 and a high-mode DCF 34.After passing through the high-mode DCF 34, light then passes through asecond mode converter 35 and into a second length of the optical fiber30. Similar to the DCF 31 of FIG. 1A, the high-mode DCF 34 exhibitschromatic dispersion which opposes that of the optical fibers 30, whilesupporting a single higher order spatial mode than that supported by theDCF 31. The mode field diameter of high-mode DCF 34 for the higher orderspatial mode is comparable to that of both optical fibers 30. Thus, themode converter 33 converts light emitted from the first optical fiber 30into the higher order spatial mode supported by the high-mode DCF 34,while the mode converter 35 reverses that conversion returning lightfrom the higher order spatial mode emitted from the high-mode DCF 34 toa lower order spatial mode for coupling back into the second opticalfiber 30. One problem exhibited by the apparatus illustrated in FIG. 1Bis that it is difficult to completely convert light from one spatialmode to another. Another problem is that it is also difficult to keeplight traveling in a single higher order spatial mode. For this reason,integrity of a signal being compensated for chromatic dispersion by theapparatus illustrated in FIG. 1B is susceptible to modal dispersion,caused by differing group velocities for light propagating in multipledifferent spatial modes.

Due to the difficulties in mode matching a DCF to various differenttypes of optical fibers 30 in the field, it is impractical to adjustchromatic dispersion exhibited by DCF's to that required by a particularoptical fiber 30. In addition, DCF's also exhibit high insertion loss.This loss of optical signal strength must be made up by opticalamplifiers. Thus, compensating for chromatic dispersion using DCF'ssignificantly increases the overall cost of an optical communicationsystem.

A different technique, shown schematically in FIG. 2, uses a chirpedfiber Bragg grating 42 to provide chromatic dispersion compensation.Differing wavelength components of a light pulse emitted from theoptical fiber 30 enter the chirped grating 42 through a circulator 41 tobe reflected back towards the circulator 41 from different sections ofthe chirped grating 42. A carefully designed chirped grating 42 cantherefore compensate for chromatic dispersion accumulated in the opticalfiber 30. The amount of chromatic dispersion provided by the chirpedgrating 42 can be adjusted by changing the stress and/or temperature ofthe grating fiber. Unfortunately, a Bragg grating reflects only a narrowband of the WDM spectrum. Multiple chirped gratings 42 can be cascadedto extend the spectral width. However, cascading multiple chirpedgratings 42 results in an expensive chromatic dispersion compensationdevice.

Yet another technique, shown schematically in FIG. 3A, employs bulkdiffraction gratings 50 for chromatic dispersion compensation.Specifically, light exiting-the transmission optical fiber 30 is firstformed into a collimated beam 51. The bulk diffraction grating 50 isthen used to generate angular dispersion (rate of diffraction anglechange with respect to the wavelength) from the collimated beam 51. Alight-returning device 52, which typically consists of a lens 53followed by a mirror 54 placed at the focal plane of the lens 53,reflects the diffracted light back onto the diffraction grating 50.Reflection of the diffracted light back onto the diffraction grating 50converts the angular dispersion into chromatic dispersion. A circulatorinserted along the path of the collimated beam 51 may be used toseparate chromatic dispersion compensated light leaving the diffractiongrating 50 from the incoming collimated beam 51. In the apparatusdepicted in FIG. 3a, the amount of chromatic dispersion may be adjustedby varying the distance between the diffraction grating 50 and the lens53, and/or the curvature of the beam-folding mirror 54. However, thebulk diffraction grating 50 produces only a small angular dispersion.Consequently, using the apparatus depicted in FIG. 3A to compensate forthe large chromatic dispersion which occurs in optical communicationsystems requires an apparatus that is impractically large.

An analogous chromatic dispersion compensation technique replaces thediffraction grating 50 with a virtually imaged phased array (“VIPA”)such as that described in U.S. Pat. No. 6,390,633 entitled “OpticalApparatus Which Uses a Virtually Imaged Phased Array to ProduceChromatic Dispersion” which issued May 21, 2002, on an application filedby Masataka Shirasaki and Simon Cao (“the '633 patent”). As illustratedin FIG. 3B, which reproduces FIG. 7 of the '633 patent, the VIPAincludes a line-focusing element, such as a cylindrical lens 57, and aspecially coated parallel plate 58. A collimated beam 51 enters the VIPAthrough the line-focusing cylindrical lens 57 at a small angle ofincidence, and emerges from the VIPA with large angular dispersion. Incombination with the light-returning device 52 illustrated in FIG. 3A,the VIPA can generate sufficient chromatic dispersion to compensate fordispersion occurring in an optical fiber transmission system.Unfortunately, the VIPA distributes the energy of the collimated beam 51into multiple diffraction orders. Because of each diffraction orderexhibits different dispersion characteristics, only one of the orderscan be used in compensating for chromatic dispersion. Consequently, theVIPA exhibits high optical loss, and implementing dispersion slopecompensation using a VIPA is both cumbersome and expensive. The VIPAalso introduces high dispersion ripple, i.e., rapid variation of residuedispersion with respect to wavelength, which renders the VIPA unsuitablefor inline chromatic dispersion compensation.

Another technique which may be used in compensating for chromaticdispersion is an all-pass filter. An all-pass filter is a device thatexhibits a flat amplitude response and periodic phase response to anincoming optical signal. Since as known to those skilled in the artchromatic dispersion is the second derivative of phase delay, anall-pass filter may therefore be used in compensating for chromaticdispersion. Typical implementations of all-pass filters in compensatingfor chromatic dispersion are Gires-Tournois interferometers and loopmirrors. An article entitled “Optical All-Pass Filters for PhaseResponse Design with Applications for Dispersion Compensation” by C.Madsen and G. Lenz published in IEEE Photonic Technology Letters, Vol.10, No. 7 at p. 944 (1998) discloses how all-pass filters may be usedfor compensating chromatic dispersion. Problems in using all-passfilters in compensating for chromatic dispersion include theirintroduction of high dispersion ripple, or an inability to producesufficient dispersion compensation for practical applications.Consequently, all-pass filters are also unsuitable for inline chromaticdispersion compensation.

Because compensating for chromatic dispersion is so important inhigh-performance optical fiber communication systems, a simpleadjustable dispersion compensator having low dispersion ripple,relatively low insertion loss, and which can compensate for variousdifferent types of chromatic dispersion exhibited by the variousdifferent types of optical fibers already deployed in fiber optictransmission systems would be highly advantageous for increasing bothbit-rate/transport-distance, and the number of WDM channels carried byan optical fiber.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a method and an apparatus which producesan adjustable amount of chromatic dispersion, and which is practical forcompensating chromatic dispersion of optical fiber systems.

An object of the present invention is to provide chromatic dispersioncompensation which exhibits low dispersion ripple.

Another object of the present invention is to provide chromaticdispersion compensation which exhibits relatively low insertion loss.

Another object of the present invention is to provide practicalchromatic dispersion compensation.

Another object of the present invention is to provide chromaticdispersion compensation which can compensate for various different typesof chromatic dispersion exhibited by the various different types ofoptical fibers that are already deployed, or which may be deployed inthe future, in fiber optic transmission systems.

Another object of the present invention is to provide chromaticdispersion compensation which increases bit-rate/transport-distance.

Another object of the present invention is to provide chromaticdispersion compensation which increases the number of WDM channels whichan optical fiber can carry.

Another object of the present invention is to provide chromaticdispersion compensation which concurrently compensates both for GVD anddispersion slope.

Another object of the present invention is to provide chromaticdispersion compensation which concurrently compensates both for GVD anddispersion slope across an entire range of wavelengths propagatingthrough an optical fiber.

Another object of the present invention is to provide chromaticdispersion compensation which is less susceptible to nonlinear effects.

Another object of the present invention is to provide chromaticdispersion compensation which does not require converting light betweendiffering spatial modes.

Another object of the present invention is to provide chromaticdispersion compensation which is less susceptible to modal dispersion.

Another object of the present invention is to provide an apparatus forchromatic dispersion compensation which occupies a comparatively smallamount of space.

Another object of the present invention is to provide cost effectivechromatic dispersion compensation for optical communication systems.

Briefly, the present invention is an optical chromatic dispersioncompensator and a method of operation thereof which is adapted forbettering performance of an optical communication system. In a preferredembodiment the chromatic dispersion compensator includes a collimatingmeans for receiving a spatially diverging beam of light which contains aplurality of frequencies as may be emitted from an end of an opticalfiber included in an optical communication system. The collimating meansconverts the spatially diverging beam of light into a mainly collimatedbeam of light that is emitted from the collimating means.

The chromatic dispersion compensator also includes an optical phaserwhich provides an entrance window for receiving the mainly collimatedbeam of light from the collimating means. The optical phaser angularlydisperses the received beam of light in a banded pattern that is emittedfrom the optical phaser. In this way the beam of light received by theoptical phaser becomes separated into bands so that light having aparticular frequency within a specific band is angularly displaced fromlight at other frequencies within that same band.

Finally, the chromatic dispersion compensator includes a light-returningmeans which receives the angularly dispersed light having the bandedpattern that is emitted from the optical phaser. The light-returningmeans reflects that light back through the optical phaser to exit theoptical phaser near the entrance window thereof.

These and other features, objects and advantages will be understood orapparent to those of ordinary skill in the art from the followingdetailed description of the preferred embodiment as illustrated in thevarious drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram depicting a prior art technique forchromatic dispersion compensation which uses a special dispersioncompensating optical fiber for reducing chromatic dispersion in anoptical communication system;

FIG. 1B is a schematic diagram depicting a prior art technique forchromatic dispersion compensation which uses mode converters and ahigh-mode dispersion compensating optical fiber for reducing chromaticdispersion in an optical communication system;

FIG. 2 is a schematic diagram depicting a prior art technique forchromatic dispersion compensation which uses a fiber Bragg grating forreducing chromatic dispersion in an optical communication system;

FIG. 3A is a schematic diagram depicting a prior art technique forchromatic dispersion compensation which uses a bulk diffraction gratingand a light-returning device for reducing chromatic dispersion in anoptical communication system;

FIG. 3B is a schematic diagram depicting a prior art technique forchromatic dispersion compensation which uses a VIPA to produce largeangular dispersions required for reducing chromatic dispersion in anoptical communication system;

FIG. 4 is a schematic diagram depicting one embodiment of a chromaticdispersion compensation apparatus in accordance with the presentinvention which includes a light-coupling means, an optical phaser, anda light-reflecting means;

FIG. 5A is a schematic diagram depicting a plan view of a prism basedlight-coupling means illustrated in FIG. 4 of one embodiment of thepresent invention;

FIG. 5B is a schematic diagram depicting a plan view of a bulkdiffraction grating based light-coupling means illustrated in FIG. 4 ofan alternative embodiment of the present invention;

FIG. 6A is a schematic diagram depicting a diffraction pattern producedby the prior art VIPA for a beam of light having a single wavelength;

FIG. 6B is a schematic diagram depicting a diffraction pattern producedby an optical phaser in accordance with the present invention fur a beamof light having a single wavelength;

FIGS. 7A, 7B, 7C, 7D, 7E and 7F are schematic diagrams depicting variousdifferent configurations for exemplary embodiments of the optical phaserall in accordance with the present invention;

FIG. 8A is a schematic diagram illustrating an embodiment of the presentinvention in which the light-returning means employs a concave mirror asthe light-focusing means;

FIG. 8B is a schematic diagram illustrating locations for thelight-focusing element and light-returning mirror with respect to theoptical phaser for one embodiment of the present invention;

FIG. 9A is a plan view schematic diagram illustrating a method forcoupling chromatic dispersion compensated light back into acommunication system in accordance with one embodiment of the presentinvention;

FIG. 9B is a plan view schematic diagram illustrating an alternativemethod for coupling light, compensated for chromatic dispersion inaccordance with the present invention, back into a communication system;

FIG. 10 is a schematic diagram illustrating intensity distributionsoccurring at the light-returning mirror of FIGS. 4, 8A and 9A byembodiments of the present invention for an incoming light beam thatcontains multiple WDM channels;

FIG. 11 is a schematic diagram illustrating various different shapes forthe light-returning mirror of FIG. 4 in accordance with the presentinvention which respectively fully compensate chromatic dispersionexhibited by various types of commercially available optical fibers; and

FIG. 12 is an eye-diagram depicting results for a simulation of a 10Gbps fiber optical transmission system containing 4000 km of opticalfiber compensated by dispersion compensators in accordance with presentinvention that are spaced at 80 km apart along the length of the opticalfiber.

DETAILED DESCRIPTION

FIG. 4 depicts an embodiment of an optical chromatic dispersioncompensator in accordance with the present invention referred to by thegeneral reference character 60. In one embodiment, the dispersioncompensator 60 includes three basic elements, a collimating means 61, anoptical phaser 62, and a light-returning means 66. The optical phaser62, explained in greater detail below, includes an entrance window 63and two parallel surfaces 64, 65. The light-returning means 66, alsoexplained in greater detail below, includes a light-focusing element 67and a curved mirror 68 that is located near the focal plane of thelight-focusing element 67.

As illustrated in FIG. 5A, a preferred embodiment of the collimatingmeans 61 includes a collimator 71 which receives a spatially divergingbeam of light emitted from an end of the optical fiber 30. As isapparent to those skilled in the art, light emitted from the end of theoptical fiber 30 may be polarized in two mutually orthogonal planes dueto the light's passage through the optical fiber 30. The collimator 71converts the spatially diverging beam of light emitted from an end ofthe optical fiber 30 into to a collimated beam 72 which, when emittedfrom the collimator 71 into free space, retains the two mutuallyorthogonal polarizations. The collimated beam 72 impinges upon abirefringent plate 73 that separates the incoming collimated beam 72into two spatially distinguishable components having perpendicularpolarizations 74, 75. Light having the polarization 74 then passesthrough a first half-wave plate 76 that rotates that light so thepolarizations of both beams lie in the same plane. The two beams oflight now both having polarizations which lie in the same plane impingeupon a prism 77 that slightly angularly disperses both beams. Bothslightly angularly dispersed beams impinge upon a second half-wave plate78 that rotates the polarizations of both beams by ninety degrees (90°).

An alternative embodiment of the collimating means 61 illustrated inFIG. 5B replaces the prism 77 with a bulk diffraction grating 77a toobtain a like amount of angular dispersion. This alternative embodimentof the collimating means 61, which includes the bulk diffraction grating77a, omits the second half-wave plate 78.

Regardless of whether the collimating means 61 uses a prism 77 or a bulkdiffraction grating 77a, as discussed in greater detail below thecollimating means 61 emits a mainly collimated beam of light. It shouldalso be noted that chromatic dispersion compensation in opticaltransport systems for which control of the dispersion slope is notcritical, such as in systems involving a limited range of wavelengths ora comparatively short optical fiber 30, the prism 77 or the bulkdiffraction grating 77a can be eliminated with little effect onperformance of the dispersion compensator 60. Moreover, those skilled inthe art will understand that the optical arrangements respectivelydepicted in FIGS. 5A and 5B can be simplified significantly if the lightexiting the optical fiber 30 has a well-defined polarization, such aslight coming directly from a laser or any other type device whichmaintains a single, planar polarization.

Light emitted from the collimating means 61 enters the optical phaser 62through the entrance window 63 to be reflected back and forth betweenthe parallel surfaces 64, 65 along the length of the optical phaser 62.The optical arrangement of either embodiment of the collimating means61, respectively illustrated in FIGS. 5A and 5B, establish polarizationsfor the beams impinging upon the entrance window 63 which areperpendicular to the incidence plane. Due to the polarization of lightimpinging upon the entrance window 63, beams of light impinging upon thesurface 65 internally within the optical phaser 62 at an angle ofincidence which is near the critical angle will be mostly reflected fromthe surface 65 even if the surface 65 lacks any optical coating.

For use in present optical communication systems, the optical phaser 62is preferably a plate of solid silicon, although it may also be made ofany other material which:

-   -   1. is transparent to light propagating through the optical        phaser 62; and    -   2. has index of refraction greater than the surrounding medium.        One of the two parallel surfaces of the optical phaser 62,        surface 64, is preferably coated with a high reflectivity film,        for example a film having a reflectivity greater than        ninety-eight percent (98%) at the wavelength of light impinging        thereon. Consequently, the surface 64 is herein referred to as        the “reflective surface.” The other surface 65 is preferably        polished, although it may also be coated with a film of partial        reflectivity, for example, with a film having a reflectivity of        approximately eighty percent (80%) at the wavelength of light        impinging thereon. The surface 65 is herein referred to as the        “detractive surface.”

One corner of the solid optical phaser 62 constituting the entrancewindow 63 is beveled. The beveled entrance window 63 is coated with ananti-reflective film to facilitate beams entering into the opticalphaser 62 therethrough. After the beams enter the optical phaser 62through the entrance window 63 at near normal incidence, they split intotwo portions at each successive impingement upon the defractive surface65 of the optical phaser 62. As explained above, most of each beamreflects internally within the optical phaser 62 upon impinging upon thesurface 65. The portion of each beam which does not reflect from thesurface 65 exits the optical phaser 62 through the surface 65 byrefraction. The configuration of the optical phaser 62 preferablyorients each beam's impingement upon the surface 65 to be at an angle ofincidence, i.e. θ, which is slightly less than the critical angle.Consequently, this configuration for the optical phaser 62 means thatrefraction of light at the surface 65 occurs near grazing emergence atan angle, i.e. φ, which is greater than forty-five degrees (45°) from anormal to the detractive surface 65. That portion of each beam reflectedat the surface 65 continues reflecting back and forth between the twoparallel surfaces 64, 65 of the optical phaser 62 with a portion of thebeam refracting out of the optical phaser 62 at each impingement of thebeam on the surface 65. Each time the beam encounters the defractivesurface 65, a small portion of the beam exits the optical phaser 62 byrefraction. Constructive interference occurs between all beams emergingfrom the surface 65 if the optical path delay between successivereflections, i.e. Δp, equals an integer multiple of the wavelength, i.e.λ, of light entering the optical phaser 62.Δp=2hn cos θ=mλ  (1)or4h²(n²−sin ²φ)=m²λ²  (2)where

-   -   n is the index of refraction of material forming the optical        phaser 62    -   θ is the angle of incidence on the surface 65 of light        reflecting internally inside the optical phaser 62    -   φ is the angle of refraction of light exiting the optical phaser        62 through the surface 65    -   h is the thickness of the optical phaser 62    -   m is the order of interference        The angular dispersion capability of the optical phaser 62, set        forth in the relationship (3) below, can be derived from        equation (2).

$\begin{matrix}{\frac{\delta\;\varphi}{\delta\;\lambda} \approx \frac{n^{2} - {\sin^{2}\varphi}}{\lambda\;\sin\;\varphi\;\cos\;\varphi}} & (3)\end{matrix}$The optical phaser 62 produces a large angular dispersion of lightexiting through the surface 65 if φ is near critical angle. A largeangular dispersion may also be realized if φ approaches normal to thesurface 65 of the optical phaser 62. The latter orientation for lightemitted from the surface 65 corresponds to the orientation of lightemitted from the parallel plate 58 of a VIPA.

Although both the optical phaser 62 and VIPA have similar angulardispersion capabilities, their diffraction patterns differsignificantly. As illustrated schematically in FIG. 6A, the beam waistinside the parallel plate 58 of the VIPA must be very small tosimultaneously reduce both the angle φ and loss of optical energy.Consequently, for a given wavelength of light λ the narrow beam waistwithin the parallel plate 58 of the VIPA produces a large angulardivergence of refracted beams. In other words, the energy of lightdiffracted by the VIPA is distributed into multiple orders. Due to thedifferent diffraction properties of the beams of different order, asstated previously for the VIPA only one of the diffraction orders may beused for dispersion compensation. Consequently, the VIPA is aninherently high-loss device. Alternatively, the beam width inside theoptical phaser 62 is similar to the thickness h of the optical phaser62. This wide beam width within the optical phaser 62 causes opticalenergy of light refracted at the surface 65 to be mainly concentrated ina single order for any beam of light at a particular wavelength asillustrated schematically in FIG. 6B.

To compensate chromatic dispersion in an optical communication systemcontaining multiple WDM channels, it is preferable to design the beamincidence angle inside the optical phaser 62, θ, in accordance with thefollowing equation (4).

$\begin{matrix}{{\cos\;\theta} = \frac{c}{2h\;\Delta\;{fn}}} & (4)\end{matrix}$where

-   -   c is the speed of light    -   Δf is the frequency separation between adjacent WDM channels.        Note that n, the index of refraction of the optical phaser 62,        is wavelength dependent. The incidence angle θ therefore varies        with wavelength. In particular, for light of each WDM channel        λ_(i), there exists a specific incidence angle θ_(i). Angular        spreading of the light beam inside the optical phaser 62 is        enabled by the angular dispersion produced by the prism 77 or        bulk diffraction grating 77a of the collimating means 61. If the        incidence angle θ is near the angle of total internal        reflection, as preferred for the current embodiment, the optical        phaser 62 not only produces large angular dispersion at a        particular wavelength as shown by relationship (3), the optical        phaser 62 also amplifies angular dispersion of the collimating        means 61. Amplification of the angular dispersion provides a        means for reducing dispersion ripple.

To reduce loss of light entering the optical phaser 62 from thecollimating means 61 and to also produce preferably only one order forany beam of light at a particular wavelength in the diffraction patternof the beam exiting the surface 65 of the optical phaser 62, or perhapsa few orders, the angular dispersion produced by the collimating means61, i.e. the collimation of the beam emitted by the collimating means61, preferably has a beam waist w_(o) in the plane of the plate that isperpendicular to the parallel surfaces 64, 65 in accordance withrelationship (5) below.w_(o)≈h sin θ  (5)where

-   -   h is the thickness of the optical phaser 62    -   θ is the angle of incidence on the surface 65 of light        reflecting internally inside the optical phaser 62        Collimating the beam of light emitted from the collimating means        61 in accordance with relationship (5) above ensures that more        than fifty-percent (50%) of the energy in the mainly collimated        beam of light impinging upon the entrance window 63 diffracts        into fewer than three (3) diffraction orders for any beam of        light at a particular wavelength in the angularly dispersed        light emitted from the optical phaser in the banded pattern.

Several alternative embodiments for the optical phaser 62 areillustrated in FIGS. 7A through 7F. In those various alternativeembodiments of the optical phaser 62, the entrance window 63 may beformed either by a beveled surface as illustrated in FIG. 4, or by aprism 82 that projects out of one of the parallel surfaces 64, 65 asillustrated in FIGS. 7D through 7F. Light entering the entrance window63 of the prism 82 reflects internally within the prism 82 beforeimpinging for a first time on one of the parallel parallel surfaces 64or 65. As illustrated for the various alternative embodiments, thereflective surface 64 may either be coated with a high-reflectivity filmor be partially transparent. If the surface 64 is partially transparent,the optical phaser 62 exhibits greater optical loss. However, for suchconfigurations of the optical phaser 62 light leaking from the surface64 may be used for performance monitoring. It should be noted that ifthe reflectivities of the parallel surfaces 64, 65 were madepolarization independent by special optical coatings, polarizationcontrol produced by the collimating means 61 for light impinging uponthe entrance window 63 of the optical phaser 62 is unnecessary.

As described previously, the preferred embodiment of the light-returningmeans 66 includes the light-focusing element 67 and a curved mirror 68placed near the focal plane of the light-focusing element 67. Thelight-focusing element 67 may be a lens as indicated in FIG. 4.Alternatively as illustrated in FIG. 8A, a concave mirror may also beused for the light-focusing element 67 in a folded configuration of thelight-returning means 66. The light-focusing element 67 is preferablylocated along the direction of the diffracted beam emitted from thesurface 65 of the optical phaser 62 at a distance, as illustrated inFIG. 8B, which is approximately one focal length, i.e. f, of thelight-focusing element 67 away from the surface 65.

In the preferred embodiment of the light-returning means 66, the lightbeams emerging from the surface 65 of the optical phaser 62 arecollected by the light-focusing element 67 for projection onto thecurved mirror 68 that is located near the focal plane of thelight-focusing element 67. Reflected back by the curved mirror 68, thebeams reverse their trajectory through the light-returning means 66, theoptical phaser 62 to exit therefrom through the entrance window 63, andproceed through the collimator 71 of the collimating means 61.Preferably, as illustrated in the plan view of FIG. 9A, light returningthrough the collimator 71 can be spatially separated from light enteringtherethrough by slightly tilting the light-focusing element 67perpendicular to a plane of symmetry of the dispersion compensator 60.Alternatively, as indicated in FIG. 9B light returning collinearlythrough the collimator 71 can also be separated from light entering thecollimator 71 by a circulator 86. While FIG. 9B illustrates thecirculator 86 as being located between the optical fiber 30 and thecollimating means 61, alternatively the circulator 86 can be insertedbetween the collimating means 61 and the optical phaser 62.

The chromatic dispersion, β, produced by the dispersion compensator 60follows a relationship (6) set forth below.

$\begin{matrix}{\beta \approx {- \frac{2\left( {n^{2} - 1} \right)^{2}f^{2}}{c\;\lambda\; R\;\cos^{2}\varphi}}} & (6)\end{matrix}$where

-   -   R is the radius of curvature of the curved mirror 68.        Note that R is defined as positive for a convex mirror and        negative for a concave mirror. For fixed diffraction angle φ and        fixed focal length f, relationship (6) indicates that the        chromatic dispersion generated by the dispersion compensator 60        is directly proportional to curvature of the curved mirror 68.        In particular, by adjusting the curvature of the curved mirror        68, it is always possible to completely cancel chromatic        dispersion of a particular optical transmission system for a        specified wavelength of light traveling therethrough.

Furthermore, the small angular dispersion introduced by the prism 77 orthe bulk diffraction grating 77a of the collimating means 61 produces abanded pattern that angularly disperses beams of light of differingwavelengths emerging from the surface 65 of the optical phaser 62. Thatis, the optical phaser 62 diffracts WDM channels having differingwavelengths of light at slightly different angles. Furthermore, lighthaving a particular frequency within each specific band of the bandedpattern is angularly displaced from light at other frequencies withinthat same band. Moreover, the banded pattern of angularly dispersedlight generated by the optical phaser 62 exhibits a rate of angularchange with respect to a center frequency within a particular band thatdiffers from the rate of angular change with respect to centerfrequencies of other bands. Consequently, as indicated schematically inFIG. 10 the light-focusing element 67 projects this banded pattern forlight of each WDM channel to a distinct location on the curved mirror 68located at the focal plane of the light-focusing element 67.

Projection of the banded pattern by the light-focusing element 67 todistinct locations on the curved mirror 68 may be exploitedadvantageously if the curved mirror 68 has a curvature which variesacross the focal plane of the light-focusing element 67. Employing acurved mirror 68 having an appropriately varying curvature permits thedispersion compensator 60 to concurrently compensate for chromaticdispersion for all WDM channels propagating through the optical fiber30. FIG. 11 displays preferred shapes for the curved mirror 68 ofexemplary embodiments of the dispersion compensator 60 that fullycompensate GVD and dispersion slope for various different types ofcommercially available optical fibers 30.

In one exemplary embodiment of the dispersion compensator 60, theoptical phaser 62 is made from a plate of silicon that is approximately1 mm thick. In accordance with the various embodiments for the opticalphaser 62 depicted in FIGS. 7A-7F, the entrance window 63 is formed onan outer surface of the plate. The reflective surface 64 has a goldcoating, and the refractive surface 65 is polished. The beam incidenceangle θ inside the optical phaser 62 is approximately sixteen degrees(16°), and the focal length of the light-focusing element 67 isapproximately 100 mm.

The dispersion compensator 60 of the present invention provides severaladvantages and distinctions of over existing dispersion compensationdevices.

First, the dispersion compensator 60 enables independent control both ofGVD and of dispersion slope. Specifically, for any optical fiber 30 of aspecified length, its GVD can be compensated by an appropriate curvatureof the folding curved mirror 68, and its dispersion slope can becompensated by appropriate curvature variations of the same foldingcurved mirror 68.

Second, the nearly collimated beam inside the optical phaser 62concentrates energy of the diffracted light into a few diffractionorders for any beam of light at a particular wavelength, resulting inbroad pass-band width and minimum throughput loss. For example, thedispersion compensator 60 of the present invention exhibits a 0.5 dBbandwidth greater than 40 GHz for a WDM system having immediatelyadjacent channels spaced 100 GHz apart.

Third, according to relationship (6), the GVD and dispersion slopeproduced by the dispersion compensator 60 change linearly with curvatureof the folding curved mirror 68. Therefore, the shape of the curvedmirror 68 that provides full GVD and dispersion slope compensation foran optical system is uniquely determined by the type of optical fiber30, and changes linearly with the length of the optical fiber 30.Accordingly, in FIG. 11 the vertical axis associate with the graphicdepiction of various curvatures for different curved mirrors 68 isnormalized to the length of the various different optical fibers 30.

Fourth, the dispersion compensator 60 can be designed with minimumdispersion slope, for example, by setting the radius of the foldingcurved mirror 68 in accordance with the following relationship (7).R cos ²φ≈cons tan t  (7)The dispersion compensator 60 equipped with a curved mirror 68 inaccordance with relationship (7) can be useful for compensatingchromatic dispersion in optical communication systems where:

-   -   1. the wavelength of the light is unstable, such as that from an        uncooled laser; or    -   2. the spectrum of the light is broad, such as that from a        directly modulated laser.

Finally, the dispersion compensator 60 introduces little dispersionripple into light propagating through an optical communication system.Therefore, the dispersion compensator 60 can be used for terminalchromatic dispersion compensation, as well as for inline chromaticdispersion compensation of long-haul fiber optical systems. For inlinechromatic dispersion compensation of long-haul fiber optical systems anumber of dispersion compensators 60 are installed at spaced apartlocations along the optical fiber 30 inline with the optical fiber 30.For example, FIG. 12 displays a eye-diagram results from a simulation ofa 10 Gbps fiber optical transmission system containing 4000 km of fibercompensated by dispersion compensators 60 of the present inventionspaced at 80 km apart along the optical fiber 30. As is apparent tothose skilled in the art, the eye-diagram of FIG. 12 exhibits littledegradation due to chromatic dispersion.

Although the present invention has been described in terms of thepresently preferred embodiment, it is to be understood that suchdisclosure is purely illustrative and is not to be interpreted aslimiting. For example, the embodiments of the dispersion compensator 60described above preferably include a prism 77 or bulk diffractiongrating 77a to provide angular dispersion of light emitted from thecollimating means 61 that impinges upon the entrance window 63. However,it is not intended for the dispersion compensator 60 as encompassed inthe following claims necessarily include such an angular dispersionelement. As stated previously, in applications of the dispersioncompensator 60 in which dispersion slope compensation is not critical,any other type of mode coupler that produces a nearly collimated beam oflight with efficient optical coupling between the optical fiber 30 andthe optical phaser 62 may be employed as the collimating means 61. Sucha coupler may simply be a standard optical collimator.

As described above, the entrance window 63 of the optical phaser 62 ispreferably coated with an antireflective film. However, it is notintended that the dispersion compensator 60 as encompassed in thefollowing claims necessarily have such a coating. The only requirementis that the entrance window 63 of the optical phaser 62 must simply bepartially transparent at the wavelength of light impinging thereon.

In the above embodiments of the present invention, the birefringentplate 73 and the half-wave plates 76, 78 linearly polarize the beam oflight received from the optical fiber 30 before suitably polarized beamsimpinge on the prism 77 and the optical phaser 62 to become angularlydispersed thereby. However, it is not intended that the dispersioncompensator 60 encompassed by the following claims be limited to usingthese specific polarization components. Instead, the dispersioncompensator 60 simply requires that an appropriately polarized beam oflight impinge on the entrance window 63 of the optical phaser 62.

Further, as described above the parallel surfaces 64, 65 of the opticalphaser 62 may be coated with films such that the correspondingreflectivities are insensitive to beam polarizations. If such coatingsare applied to the parallel surfaces 64, 65, then polarization of thebeam of light impinging upon the entrance window 63 need not becontrolled, and the polarization control components, e.g. thebirefringent plate 73 and the half-wave plates 76, 78, may be eliminatedfrom the collimating means 61. Analogously, to increase opticalefficiency an antireflective coating may be advantageously applied tothe light-focusing element 67 to reduce loss of light passing through alens. For optical efficiency it is also advantageous if the curvedmirror 68 have a highly reflective coating.

As described above, the preferred spacing between the surface 65 of theoptical phaser 62 and the light-focusing element 67 equals the focallength, f, of the light-focusing element 67. However, the dispersioncompensator 60 encompassed by the following claims is not limited tothat specific geometry. Instead, the distance between the focusingelement 66 and the surface 64 of the phaser 61 may be set to any value.As described in greater detail below, that distance may, in fact, evenbe adjustable.

In general, the chromatic dispersion produced by the apparatus of thepresent invention is related to its geometry by the followingrelationship (8).

$\begin{matrix}{\beta \propto \left( {f - u + \frac{f^{2}}{R}} \right)} & (8)\end{matrix}$where

-   -   u is the distance from the surface 65 of the optical phaser 62        to the light-focusing element 67 along the optical axis of the        light-focusing element 67    -   f the focal length of the light-focusing element 67    -   R the radius of curvature of the folding curved mirror 68.        A tunable dispersion compensator 60 in accordance with the        present invention can be implemented by adjusting u, or R or        both u and R. For the adjustment of u, a preferred embodiment of        the dispersion compensator 60 is to place the light-returning        means 66 on a translation stage, as indicated in FIG. 4 by an        arrow 69. Alternatively, the curved mirror 68 can be made with        adjustable curvature.

There exist numerous different ways which may be employed to make thecurvature of the curved mirror 68 adjustable. One way is to applyelastic bending forces to the curved mirror 68 in the directionindicated in FIG. 4. by arrows 70 Such bending forces may be generatedmechanically such as by push screws. Alternatively, the forces may alsobe generated electrostatically or electromagnetically such as by a microelectro-mechanic system. The curvature of the curved mirror 68 may alsobe adjusted thermally if the mirror is formed from a bi-metallicmaterial. Optimal mirror shapes may be achieved by forming the curvedmirror 68 to have varying stiffness, or by applying bending forces atmultiple locations on the curved mirror 68, or by a combination of bothtechniques. Translating a curved mirror 68 having uneven curvatures thatis located near the focal plane of the light-focusing element 67transverse to the optical axis thereof, i.e. translating along the focalplane of the light-focusing element 67, also adjusts the curvature ofthe curved mirror 68. The curvature of the curved mirror 68 may also beadjusted by replacing a curved mirror 68 having a particular shape withanother one having a different shape.

Consequently, without departing from the spirit and scope of theinvention, various alterations, modifications, and/or alternativeapplications of the invention will, no doubt, be suggested to thoseskilled in the art after having read the preceding disclosure.Accordingly, it is intended that the following claims be interpreted asencompassing all alterations, modifications, or alternative applicationsas fall within the true spirit and scope of the invention.

1. An optical chromatic dispersion compensator adapted for betteringperformance of an optical communication system comprising: a collimatingmeans for receiving a spatially diverging beam of light which contains aplurality of frequencies as may be emitted from an end of an opticalfiber included in an optical communication system, the collimating meansalso converting the received spatially diverging beam of light into amainly collimated beam of light that is emitted from the collimatingmeans; an optical phaser which provides an entrance window for receivingthe mainly collimated beam of light from the collimating means and forangularly dispersing the received beam of light in a banded pattern thatis emitted from the optical phaser, whereby the received beam of lightbecomes separated into bands so that light having a particular frequencywithin a specific band is angularly displaced from light at otherfrequencies within that same band; and a light-returning means whichreceives the angularly dispersed light having the banded pattern that isemitted from the optical phaser, and for reflecting that light backthrough the optical phaser to exit the optical phaser near the entrancewindow thereof.
 2. The compensator of claim 1 wherein the mainlycollimated beam of light emitted from the collimating means has adivergence which ensures that more than fifty-percent (50%) of energy inthe mainly collimated beam of light impinging upon the entrance windowdiffracts into fewer than three (3) diffraction orders for any beam oflight at a particular wavelength in the angularly dispersed lightemitted from the optical phaser in the banded pattern.
 3. Thecompensator of claim 1 wherein light enters the optical phaser throughthe entrance window at near normal incidence.
 4. The compensator ofclaim 1 wherein the entrance window of the optical phaser is at leastpartially transparent to light impinging thereon.
 5. The compensator ofclaim 1 wherein the light-returning means includes a light-focusingmeans and a mirror disposed near a focal plane of the light-focusingmeans, the light-focusing means collecting the angularly dispersed lighthaving the banded pattern emitted from the optical phaser for projectiononto the mirror, the mirror reflecting light impinging thereon backtowards the light-focusing means.
 6. The compensator of claim 5 whereinthe light-focusing means projects to a distinct location on the mirroreach band in the banded pattern of angularly dispersed light generatedby the optical phaser.
 7. The compensator of claim 5 wherein a distancebetween the light-focusing means and the optical phaser is adjustable.8. The compensator of claim 5 wherein the mirror is curved.
 9. Thecompensator of claim 8 wherein curvature of the mirror is adjustable.10. The compensator of claim 9 wherein curvature of the mirror isadjusted by bending the mirror.
 11. The compensator of claim 10 whereinforce for bending the mirror is selected from a group consisting ofmechanical, electrical, magnetic and thermal.
 12. The compensator ofclaim 9 wherein the mirror has multiple curvatures, and curvature of themirror is adjusted by translating the mirror.
 13. The compensator ofclaim 9 wherein the mirror is replaceable, and curvature of the mirroris adjusted by replacing the mirror with another mirror having adifferent curvature.
 14. The compensator of claim 1 wherein the opticalphaser is made from a plate of material having two parallel surfacesbetween which light after entering the optical phaser through theentrance window reflects, and with the entrance window being formed onan outer surface of the plate.
 15. The compensator of claim 14 whereinthe entrance window is formed by a beveled edge of the plate.
 16. Thecompensator of claim 14 wherein the entrance window is formed by a prismwhich projects out of one of the two parallel surface surfaces of theoptical phaser, and light entering the optical phaser through theentrance window undergoes internal reflection within the prism beforeimpinging upon one of the two parallel surface surfaces.
 17. Thecompensator of claim 14 wherein one of the two parallel surface surfacesof the optical phaser is partially transparent to allow a portion oflight impinging thereon to exit the optical phaser.
 18. The compensatorof claim 17 wherein light emitted from the optical phaser through thepartially transparent surface detracts at an angle which exceedsforty-five degrees (45°) from a normal thereto.
 19. The compensator ofclaim 1 wherein the optical phaser is made from a material having anindex of refraction which is greater than the index of refraction ofmedium surrounding the optical phaser.
 20. A chromatic dispersioncompensation method that is adapted for bettering performance of anoptical communication system comprising the steps of: collimating into amainly collimated beam of light a spatially diverging beam of lightwhich contains a plurality of frequencies as may be emitted from an endof an optical fiber included in an optical communication system;impinging the mainly collimated beam of light onto an entrance window ofan optical phaser for angularly dispersing the mainly collimated beam oflight into a banded pattern emitted from the optical phaser whereby themainly collimated beam of light becomes separated into bands so thatlight having a particular frequency within a specific band is angularlydisplaced from light at other frequencies within that same band; andreflecting the angularly dispersed light back through the optical phaserto exit the optical phaser near an entrance window thereof.
 21. Themethod of claim 20 wherein the mainly collimated beam of light has adivergence which ensures that more than fifty-percent (50%) of energy inthe mainly collimated beam of light impinging upon the entrance windowdiffracts into fewer than three (3) diffraction orders for any beam oflight at a particular wavelength in the angularly dispersed lightemitted from the optical phaser in the banded pattern.
 22. The method ofclaim 20 wherein a light-returning means for reflecting the angularlydispersed light back through the optical phaser includes alight-focusing means and a mirror disposed near a focal plane of thelight-focusing means, the method further comprising the steps of: thelight-focusing means collecting the angularly dispersed light having thebanded pattern emitted from the optical phaser for projection onto themirror; and the mirror reflecting light impinging thereon back towardsthe light-focusing means.
 23. The method of claim 22 wherein thelight-focusing means projects to a distinct location on the mirror eachband in the banded pattern of angularly dispersed light generated by theoptical phaser.
 24. The method of claim 22 further comprising the stepof adjusting a distance which separates the light-focusing means fromthe optical phaser.
 25. The method of claim 22 further comprising a stepof adjusting a curvature of the mirror.
 26. The method of claim 25wherein curvature of the mirror is adjusted by bending the mirror. 27.The method of claim 26 wherein force for bending the mirror is selectedfrom a group consisting of mechanical, electrical, magnetic and thermal.28. The method of claim 25 wherein the mirror has multiple curvatures,and curvature of the mirror is adjusted by translating the mirror. 29.The method of claim 25 wherein the mirror is replaceable, and curvatureof the mirror is adjusted by replacing the mirror with another mirrorhaving a different curvature.
 30. The method of claim 20 wherein lightis emitted from the optical phaser through a partially transparentsurface thereof, the emitted light being defracted at an angle whichexceeds forty-five degrees (45°) from a normal to the partiallytransparent surface.
 31. A chromatic dispersion compensation method thatis adapted for bettering performance of an optical communication systemcomprising the steps of: collimating into a mainly collimated beam oflight a spatially diverging beam of light which contains a plurality offrequencies as may be emitted from an end of an optical fiber includedin an optical communication system; impinging the mainly collimated beamof light onto an entrance window of an optical phaser (62), the opticalphaser (62) having an entrance window (63) for receiving a beam oflight, and having opposed parallel surfaces (64, 65) one of which ishighly reflective and one of which is at least partially transmissive,and wherein the optical phaser (62) is arranged such that the angle ofincidence of the beam's impingement upon the partially transmissivediffractive surface (65) is slightly less than the angle of totalinternal reflection; angularly dispersing in the optical phaser themainly collimated beam of light into a banded pattern emitted from theoptical phaser whereby the mainly collimated beam of light becomesseparated into bands so that light having a particular frequency withina specific band is angularly displaced from light at other frequencieswithin that same band; and reflecting the angularly dispersed light backthrough the optical phaser to exit the optical phaser near an entrancewindow thereof.
 32. The method of claim 31 wherein the mainly collimatedbeam of light has a divergence which ensures that more thanfifty-percent (50%) of energy in the mainly collimated beam of lightimpinging upon the entrance window diffracts into fewer than three (3)diffraction orders for any beam of light at a particular wavelength inthe angularly dispersed light emitted from the optical phaser in thebanded pattern.
 33. The method of claim 31 wherein a light-returningmeans for reflecting the angularly dispersed light back through theoptical phaser includes a light-focusing means and a mirror disposednear a focal plane of the light-focusing means, the method furthercomprising the steps of: the light-focusing means collecting theangularly dispersed light having the banded pattern emitted from theoptical phaser for projection onto the mirror; and the mirror reflectinglight impinging thereon back towards the light-focusing means.
 34. Themethod of claim 33 wherein the light-focusing means projects to adistinct location on the mirror each band in the banded pattern ofangularly dispersed light generated by the optical phaser.
 35. Themethod of claim 33 further comprising the step of adjusting a distancewhich separates the light-focusing means from the optical phaser. 36.The method of claim 33 further comprising a step of adjusting acurvature of the mirror.
 37. The method of claim 36 wherein curvature ofthe mirror is adjusted by bending the mirror.
 38. The method of claim 37wherein force for bending the mirror is selected from a group consistingof mechanical, electrical, magnetic and thermal.
 39. The method of claim36 wherein the mirror has multiple curvatures, and curvature of themirror is adjusted by translating the mirror.
 40. The method of claim 36wherein the mirror is replaceable, and curvature of the mirror isadjusted by replacing the mirror with another mirror having a differentcurvature.
 41. The method of claim 31 wherein light is emitted from theoptical phaser through a partially transmissive surface thereof, theemitted light being defracted at an angle which exceeds forty-fivedegrees (45°) from a normal to the partially transmissive surface. 42.An optical chromatic dispersion compensator adapted for betteringperformance of an optical communication system comprising: an opticalphaser (62) having an entrance window (63) for receiving a beam oflight, and having opposed parallel surfaces (64,65) one of which ishighly reflective and one of which is at least partially transmissivefor angularly dispersing the received beam of light in a banded patternthat is emitted from the optical phaser, through the partiallytransmissive diffractive surface (65), whereby the received beam oflight becomes separated into bands so that light having a particularfrequency within a specific band is angularly displaced from light atother frequencies within the same band; and a light-returning means (66)which receives the angularly dispersed light having the banded patternthat is emitted from the partially transmissive diffractive surface (65)of the optical phaser, and for reflecting that light back through theoptical phaser to exit the optical phaser near the entrance windowthereof; characterized by: a collimating means (61) for receiving aspatially diverging beam of light which contains a plurality offrequencies as may be emitted from an end of an optical fiber includedin an optical communication system, the collimating means alsoconverting the received spatially diverging beam of light into a mainlycollimated beam of light that is emitted from the collimating means andimpinges upon the entrance window (63) of the optical phaser (62); andwherein the optical phaser (62) is arranged such that the angle ofincidence of the beam's impingement upon the partially transmissivediffractive surface (65) is slightly less than the angle of totalinternal reflection.
 43. The compensator of claim 42 wherein the mainlycollimated beam of light emitted from the collimating means (61) has adivergence which ensures that more than fifty-percent (50%) of energy inthe mainly collimated beam of light impinging upon the entrance windowdiffracts into fewer than three (3) diffraction orders for any beam oflight at a particular wavelength in the angularly dispersed lightemitted from the optical phaser (62) in the banded pattern.
 44. Thecompensator of claim 42 wherein light enters the optical phaser (62)through the entrance window at near normal incidence.
 45. Thecompensator of claim 42 wherein the entrance window of the opticalphaser (62) is at least partially transparent to light impingingthereon.
 46. The compensator of claim 42 wherein the light-returningmeans (66) includes a light-focusing means (67) and a mirror (68)disposed near a focal plane of the light-focusing means (67) collectingthe angularly dispersed light having the banded pattern emitted from theoptical phaser for projection onto the mirror, the mirror (68)reflecting light impinging thereon back towards the light-focusingmeans.
 47. The compensator of claim 46 wherein the light-focusing means(67) projects to a distinct location on the mirror (68) each band in thebanded pattern of angularly dispersed light generated by the opticalphaser (62).
 48. The compensator of claim 46 wherein a distance betweenthe light-focusing means (67) and the optical phaser (62) is adjustable.49. The compensator of claim 46 wherein the mirror (68) is curved. 50.The compensator of claim 49 wherein curvature of the mirror (68) isadjustable.
 51. The compensator of claim 50 wherein curvature of themirror is adjusted by bending the mirror.
 52. The compensator of claim51 wherein force for bending the mirror is selected from a groupconsisting of mechanical, electrical, magnetic and thermal.
 53. Thecompensator of claim 50 wherein the mirror (68) has multiple curvatures,and curvature of the mirror is adjusted by translating the mirror. 54.The compensator of claim 50 wherein the mirror (68) is replaceable, andcurvature of the mirror is adjusted by replacing the mirror with anothermirror having a different curvature.
 55. The compensator of claim 42wherein the optical phaser (62) is made from a plate of material havingtwo parallel surfaces (64, 65) between which light after entering theoptical phaser through the entrance window reflects, and with theentrance window being formed on an outer surface of the plate.
 56. Thecompensator of claim 55 wherein the entrance window is formed by abevelled edge of the plate.
 57. The compensator of claim 55 wherein theentrance window is formed by a prism which projects out of one of thetwo parallel surfaces (64, 65) of the optical phaser, and light enteringthe optical phaser through the entrance window undergoes internalreflection within the prism before impinging upon one of the twoparallel surfaces.
 58. The compensator of claim 42 wherein light emittedfrom the optical phaser (62) through the partially transparent surface(65) diffracts at an angle which exceeds forty-five degrees (45°) from anormal thereto.
 59. The compensator of claim 42 wherein the opticalphaser (62) is made from a material having an index of refraction whichis greater than the index of refraction of medium surrounding theoptical phaser (62).